Prior art muldems have been designed for converting between DS3 (approximately 44 megahertz) transmission line signals and one or more of the lower frequency transmission line signals such as DS2 (approximately 6 megahertz), DS1 (approximately 1.5 megahertz) and DS1C (approximately 3 megahertz). In these system concepts, there is always the need for some type of redundancy in the event that one or more parts in the muldem fails. One method is complete redundancy so that any path can be switched into operation as a replacement for the failed component. However, in view of the costs of complete redundancy, especially where there are many demultiplexer, multiplexer and switch modules, etc., of each given type in a box, a single standby unit or module for each type of operational module is used and when an operational module fails or provides an indication of failure, the standby identical module is switched into operation.
It has been found that quite often the apparent failure is merely due to momentary external conditions and the "failed" module is actually operating properly. Thus, there is a requirement that the "failed" module be tested to determine whether the module actually failed or the decision to switch was based on the momentary external conditions. If it is found that the module has not failed, then it is desirable to return operation to normal conditions so that the standby module can be utilized to replace other modules believed to have failed. Otherwise, the spare module would not be available for a true failure.
As a consequence of having standby modules, it is necessary for proper operation of the system to periodically test the standby modules to insure that they are actually available for service. Thus, a properly designed digital muldem provides means of testing both the failed modules and the standby modules used to design the system.
Prior art test devices have used a pseudorandom generator to generate a data stream that is passed through the module being tested and the output of the module is checked to see whether or not the data is passed in a standardized manner. It has been found that a pseudorandom pattern of data does not necessarily generate the same failure mode as does true data. In other words, a module can test operational with a given pseudorandom data pattern and still not pass data, in some other data stream configuration, correctly.
The present invention illustrates a design method for constructing the digital muldem wherein the muldem is designed in two separate stages. The low speed muldem portion provides circuitry to convert the low speed customer interface whether it be DS1, DS1C or DS2 signals through an appropriate plurality of muldems to a common intermediate speed or data occurrence frequency. The signals are then switched or routed to a high speed muldem or its spare which converts from the common frequency to the DS3 frequency while multiplexing the data. In the present invention, economies have been realized by making the common frequency signal the same as the DS2 frequency and format. The low speed muldem spares can be used to replace any one of the identical type (DS1, DS1C or DS2) operational low speed muldems.
In the present invention, data from a low speed active circuit is applied to a failed module for multiplexing as if the failed module were operational, as well as to the spare or other active module. However, the output of the failed module is rerouted from the output of the demultiplexer to the input of the multiplexer in a loopback configuration. (This loopback is from multiplexer to demultiplexer in the high speed circuit modules.) The circuitry for providing the loopback disconnects the failed module from the customer circuitry and thus the customer is isolated from the failed module. Since the demultiplexing and multiplexing of the signal in the loopback configuration produces identical data supplied to and received from the looped-back module in a correctly operating module, a comparator can be used to compare, on a bit by bit basis, the signals input to and output from the failed module thereby ascertaining whether or not the module has actually failed, or the apparent failure, as previously detected, was merely due to some momentary external condition. The same approach can be used for testing the high speed muldem and any of the spares since all of these units have a common frequency, a single comparator circuit can be used to test all of the muldems regardless of their multiplexing frequency.
It is thus an object of the present invention concept to illustrate a method for testing muldems for operability in a muldem switching system.
Another object of the present invention is to present a method whereby a muldem can be designed to simplify the circuitry required by eliminating one or more pseudorandom generators and using, instead, actual data for testing. The invention also includes using a common intermediate frequency in the apparatus so that testing of all of the different frequency muldems can be accomplished using a single test (bit comparator) device.